The present invention relates to a system for controlling the lighting of lamps such as an exposing lamp and a fixing heater lamp in an image forming device such as a copier and a facsimile.
In conventional image forming devices such as a copier and a facsimile, there has been used such a lamp control system that compares a feed-back voltage to be applied to a lamp with a reference voltage to obtain a corrected output and controls an energy of the power supply to the lamp according to the obtained corrected output. The control system starts energizing of a lamp for example in a copying machine as soon as it received a copying start command. In some types of the machines, lamps are energized simultaneously with turning-on the power supply of the machines to confirm the normal operations of the machine portions. In this case, the lamps are checked for deterioration or breakage by, e.g., reading exposure light amount by an automatic exposure (AE) sensor. It is also determined whether an optical system can be normally set into a home position. In most cases of practice, an amount of an electric energy to be supplied to a lamp for an initial energizing period is equal to that to be supplied in a period of the stable copying operation. This may, however, produce a large rush current in an initial energizing period, resulting in breakage of switching elements such as transistors and triacs for controlling the lighting of the lamp.
Japanese Laid-open Patent Publication No. 4-10634 proposes phase control of a lamp driving voltage by gradually increasing electric current to the lamp for an initial energizing period. A summary of this prior-art lamp control system for an image forming device will be described.
A waveform of an alternating power-supply voltage (commercial electric power source of AC100 V) is full-wave rectified. A zero cross-point signal represents a zero cross-point detected on the alternating voltage waveform. When a copy operation starting command is given or an electric power circuit is turned on, a signal requesting the lighting of a lamp is input, and, therefore, a stop signal is applied to a switching element. Namely, the stop signal is output at a timing that lagged by a conduction angle .beta..sub.i (i=0, 1 . . . ) from the zero cross-point. A voltage of the conducting-angle portion .beta..sub.i of the full-wave rectified waveform is applied to the lamp and phase control is carried out. A lamp driving voltage V.sub.i (i=0, 1 . . . ) at which a lamp driving current i.sub.i is fed to the lamp.
When regarding a half-wave of the alternating voltage waveform as one cycle, as the conduction angle .beta..sub.i gradually increased every cycle as .beta..sub.0 &lt;.beta..sub.1 &lt;.beta..sub.2 &lt;.beta..sub.3 &lt;.beta..sub.4 &lt;.beta..sub.5 &lt; . . . &lt;.beta..sub.n, the voltage V.sub.i applied to the lamp is gradually increased as V.sub.0 &lt;V.sub.1 &lt;V.sub.2 &lt;V.sub.3 &lt;V.sub.4 &lt;V.sub.5 &lt; . . . &lt;V.sub.n. Accordingly, the lamp driving current i.sub.i is also gradually increased as i.sub.0 &lt;i.sub.1 &lt;i.sub.2 &lt;i.sub.3 &lt;i.sub.4 &lt;i.sub.5 &lt; . . . &lt;i.sub.n. This is so called "soft start" of the lamp for the initial energizing period. In this case, rush currents of a large peak value for initial energizing period can be eliminated, so the switching elements such as transistors for controlling the lighting of the lamp can be reliably protected from being damaged by inrush currents. As soon as the initial conducting period ceased and a normal copying period began, the conduction angle .beta..sub.c becomes constant and the lamp driving voltage and current to be stable at constant levels V.sub.c and i.sub.c respectively, thus a stable state begins.
The above-mentioned prior-art lamp-control system for the image forming device (Japanese Laid-open Patent Publication No. 4-10634) is, however, a relatively large and expensive because of using a full-wave rectifier therein.
Accordingly, a method of driving a lamp without using the full-wave rectifier has been proposed, which will be described.
When a command for starting a copying operation is given or an electric power circuit of a copying machine is turned on, a signal requesting the lighting of a lamp is input and, then, a trigger signal is applied to a bi-directional switching element such as a triac. Namely, the trigger signal is output at a time lag of a firing angle .alpha..sub.i (i=0, 1 . . . ) in respect with the zero cross-point of the alternating voltage waveform. Consequently, a voltage corresponding to the conducting-angle portion .beta..sub.i of the alternating voltage waveform is applied to the lamp and phase control is carried out. With a subsequent zero cross-point signal, the lamp driving current i.sub.i drops to zero. When a lamp driving voltage V.sub.i (i=0, 1 . . . ) is applied to the lamp, a lamp driving current i.sub.i flows the lamp.
The conduction angle .beta..sub.i begins at a timing of rising start of a zero cross-point signal whereas the conduction angle .beta..sub.i begins with a lag from the rising start timing of zero-cross-point signal by a firing angle .alpha..sub.i. Since both cases realize substantially equivalent phase control irrespective of the above-mentioned difference, the above-mentioned method is preferably applied in practice.
When counting a half-wave of the alternating voltage waveform as one cycle, as the firing angle .alpha..sub.i is gradually decreased every cycle as .alpha..sub.0 &lt;.alpha..sub.1 &lt;.alpha..sub.2 &lt;.alpha..sub.3 &lt;.alpha..sub.4 &lt;.alpha..sub.5 &lt; . . . &lt;.alpha..sub.n, the conduction angle .beta..sub.i is gradually increased every cycle as .beta..sub.0 &lt;.beta..sub.1 &lt;.beta..sub.2 &lt;.beta..sub.3 &lt;.beta..sub.4 &lt;.beta..sub.5 &lt; . . . &lt;.beta..sub.n and the voltage V.sub.i applied to the lamp is gradually increased as V.sub.0 &lt;V.sub.1 &lt;V.sub.2&lt;V.sub.3 &lt;V.sub.4&lt;V.sub.5 &lt; . . . &lt;V.sub.n. Accordingly, the lamp driving current i.sub.i is also gradually increased as i.sub.0 &lt;i.sub.1 &lt;i.sub.2&lt;i.sub.3 &lt;i.sub.4 &lt;i.sub.5 &lt; . . . &lt;i.sub.n. Thus, rush currents of a large peak value in initial lamp-energizing period can be eliminated, so the switching elements such as transistors for controlling the lighting of the lamp can be reliably protected from being damaged by the inrush currents. As the initial conducting period ceased and a normal copying period begin, the conduction angle .beta..sub.c becomes constant and the lamp driving voltage and current are stable at constant levels V.sub.c and i.sub.c respectively, thus a stable state begins.
In this case, the system may be compact and inexpensive since it does not need for using a full-wave rectifier.
In the prior art lamp control system, when gradually increasing the lamp driving voltage V.sub.i (i=0,1 . . . ) little by little, since the lamp driving voltage is gradually increased for every cycle, the polarity of the lamp driving voltage V.sub.i is altered from positive to negative or vice versa for every cycle of half-wave of the alternating voltage waveform. Noise components in positive and negative voltage are different from each other in levels, so electromagnetic noises can not cancel out each other and a large noise appears at a plug socket for supply alternating current. This may not satisfy recently established regulations for protecting external appliances against external noise and disturbance. Furthermore, these noises occurring for the initial conducting period may cause an image forming device to erroneously stop in operation or voluntarily start copying operation. To avoid such erroneous operations of the device, there arises the necessity of using a noise reducing circuit that may lose the economical merit attained by eliminating the use of the full-wave rectifier.